High efficiency modulator



Nov. 8, 1949 J. F. BELL 2,487,212

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Nov. 8, 1949 J. F. BELL HIGH EFFICIENCY MODULATOR 3 Sheets-Sheet 2 Q In Filed June 19, 1946 JOHN F. BELL INVENTOR.

Hls ATTORNEY 3 Sheets-Sheet 3 Filed June 19, 1946 llilJ 5:05-52 Patented Nov. 8, 1949 2,487,212 HIGH EFFICIENCY MODULATOR John F. Bell, Glenview, 11L, assignor to Zenith Radio Corporation, a c

Application June 19,

orporation of Illinois 1946, Serial No. 677,697

Claims. (Cl. 332-43) This invention relates to carrier wave signal transmitting apparatus and more specifically to mum overall efficiency.

In general, there exist grid modulated radio frequency amplifiers having two channels, the

network. The final amplifier called the "carrier channel, operates throughout the modulation range.

from that stage.

Such an arrangement was described by F. E. Terman in the Proceedings of Radio Engineers for August, 1938.

Initter, in addition to increasing the fidelity of the transmitted signals.

It is another object of this invention to provide a signal modulated carrier wave transmitter having increased overall efficiency.

A still further object of this invention is to provide such a transmitter of signal information which transmits that information with maximum fidelity.

These improvements make the signal transmitter especially adapted to the transmission of tele- Therefore, this invention will be discussed in terms of a transmitter of video signals although the applicability of the invention is not so confined.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present inlated radio transmitter embodying the features 01' this invention;

Figure 2 is a detailed circuit diagram of it portion of the transmitter of Figure 1:

Figure 3 is a drawing of an element of the circuit of Figure 2; and

Figure 4 is a detailed circuit diagram sentially constant.

In combination with stepdown transformers 5 and 6, driver amplifiers 3 and 4 form a. source modulating signal.

By incorporating limiting circuits in carrier modulator i it is made to supply only those moduis embodied in a blackvideo" transmitter, carrier amplifier i transmits signals corresponding to the white to grayportions of the television picture. Peak final amplifier 2 transmits the gray to black portions of the television picture, and in addition, transmits the synchronization impulses.

As a result of this amplitude splitting of the modulating signal, each of the modulating channels operates over a narrowed dynamic range, with the result that each operates with a considerably higher average efliciency than could be realised with a single channel amplifier. Furthermore, the linearity of power output with modulating power is retained and amplitude distortion is prevented.

The grid limiting circuits assure a constant maximum dynamic modulating signal range and thus prevent the distortion of the modulating signals which occurs when the power output from final amplifiers I and 2 corresponding to a given reference point on the modulating signal is not constant from signal to signal.

Final amplifiers l and 2 are coupled together by a compact transmission line impedance inverter 9 and peak final amplifier 2 is biased so it remains inoperative until the level lating signals exceeds approximately 50% of the maximum amplitude of the modulating signals, with the result that both stages operate at relatively high efilciency and the linearity of power output with modulation is retained. Amplitude disortion of the modulating signal is thus prevented.

In Figure 2, in detail, radio frequency voltage at the desired frequency from an appropriate source II) is applied simultaneously to inductance H and transmission line l2. age applied to grids l3 and It of vacuum tubes ii and It from grid coil I1 is in time phase quadrature with the voltage applied to grids l8 and IQ of vacuum tubes 20 and 21 through link 22 and grid coil 23, because the electrical length of transmission line I2 is one-quarter wavelength or 90 electrical degrees at-the carrier frequency. These excitation voltages are of sufilcient amplitude so that they drive theegrids of amplifier tubes l5, I6, 20 and 2| in class C operation when the maximum load appears across the tank circuit of amplifiers 3 and 4. Vacuum tubes l5, l6 and 20 and 2! are connected in a balanced or push-pull arrangement. They are biased to approximately twice the plate current cut-off voltage by grid current flowing through resistors 24 and 25, respectively. Grid bias source 26 provides a protective, relatively small, fixed bias which supplements the self-bias.

The continuous voltage obtained from plate voltage source 21 is applied to plates 28, 29, an and 3! of vacuum tubes IE, it, 20 and 2i respectively. Y

Plate inductance 32 resonates with the inherent stray capacitance 33 existing between plates 23 and 29 to provide a purely resistive impedance at the carrier frequency. Similarly. Plate inductance 34 resonates with the stray capacitance 35 between plates at and ti at the carrier frequency.

Cathode 33 or vacuum tube i and cathode 31 of vacuum tube 18 are heated by current from transformer 38 to cause electron emission. Similarly, cathodes 39 and It of vacuum tubes 20 and 2 l respectively, are heated by current from transformer ll.

Condensers l2 and 43 by-pass radio frequency currents around transformer 38. Similarly, conof the modu- The excitation voltdensersllandllby-passradion-eouencycurrents around transiormer 4| and prevent degeneration.

coleandllneutralizethegridto vacuum tubes 15 and It, respecself-excited oscillation of the similarly, congrid to plate or its equivalent, one-quarter wavelength long at the carrier frequency, effectively terminated by a short circuit because of the negligible reactance of condensers 52 and 53 at that frequency.

The impedance looking toward carrier driver 3 decreases as conductors 54 and 55 are connected to transformer 5 nearer and nearer to its short circuiting termination. The exciting voltages impressed between cathode 56 of vacuum tube 51 and cathode is of vacuum tube 59 correspondingly decrease. The combination of selfbiased class C amplifier 3, which itself is capable of supplying current to maintain a nearly constant voltage across a varying load, with the impedance step-down transformer 5, provides a source of constant exciting voltage for amplifier I, despite variations in the load presented to the combination by amplifier l as it is grid modulated throughout the dynamic range of the modulating voltages. Similarly, conductors 60 and GI are connected to a low impedance point on peak step-down transformer 6 and impress an exeiting voltage of constant amplitude between cathode 62 of vacuum tube 53, cathode B4 of vacuum tube 65, and ground.

Inductances 6B and 51, effectively in parallel, resonate with the reactance of stray capacitance 68 at the carrier frequency. Since conductors 54 and 55 are very short, the input capacitance to conductors 54 and 55 measured at their junction with step-down transformer 5 is extremely small and the broad band characteristics of the source of driving current for amplifier 1 are retained.

The use of two inductances 61 and (it permits the introduction of filament power for tubes 51 and 59 from transformer Gil without introducing hum modulation of the output signal from amplifier l. Condensers Ill by-pass radio frequency current across filament-cathode 56 and 5B of amplifier tubes 51 and 53, respectively,

Correspondingly,-inductances 1i and 12, effectively in parallel because of the low reactance of by-pass condensers 13 and 14 to carrier frequency currents, resonate with stray capacitance 15 at the carrier frequency.

Cathode heating power is supplied by transformer 16 through inductances 1| and 12 to amplifier tubes 63 and 65.

Modulating voltages are impressed on grids 11, 18, 19, and of tubes 51, 59, 63 and 65, respectively. These grids are at ground potential as far as voltages at the carrier frequency are concerned. Screen grids M, 82, 83 and 84 are directly grounded.

By utilizing for amplifier B5, tetrodes which have by their construction inherently low interelectrode capacitance, and low electrode-space capacitance, and, further, operating those tubes as grounded grid amplifiers, the cathode to ground, plate to ground, and plate tubes 51, 59, B3 and to plate capacitances are kept extremely low, thus increasing the spectrum 01' frequencies over terminating impedance on the line.

explained, up to carrier level (25% of full power) amplifier 2 is inoperative. load terminating inverter 9 is retained and contrast in the signal is not lost.

line, as given by the equation is increased rapidly. The velocity of propagation in h propagation. and f is 15 V reduces the velocity by means of and fed in-phase to tubes Hi8 and I0! input amplifier I08 09, respectively.

Automatic self-bias is developed across resistors H0 and H are included 4 and carrier input amplifier til tips are amplified.

An output signal of negative polarity is taken peak input amplifier I 08 and is age impressed on plate 8a of diode H7 tiometer H9. Wh

voltage appears across resistor amplification of any porti Similarly. the signal of negative polarity from Similarly, carrier rectifier Ill includes step-up plate III of vacuum tube I01 are coupled through transiormer Ill, rectifier tube its, across which condenser I2: and grid clipping resistor I23 to the high voltage from transiormer its is imgrid I of intermediate amplifier tube I". pressed, voltage regulator I60, which maintains The output trorn amplifier its is also impressed a constant output voltage from the rectifier. and on cathode I26 oi diode I21 and the negative pofilter condenser IBI, which aids in eliminating tential on anode I2! is adjusted by means oi poripple from the continuous voltage delivered by tentiometer I29 so that the synchronization tips the rectifier. The voltage appearing across filter lie beyond the cut-off voltage for amplifier tube condenser Itl is applied between screen grid I43 I23, and only the gray and white portions of the of carrier modulator I40 and cathode I62 of that television picture are amplified. same tube.

The output signals from plate In oi int Carrier grid current meter IE3 is connected bedmt amplifier mm Hg are of positive polarity tween grid load resistor I64 and the negative terand are coupled through condenser iii and rem n l of bias supp y to m n the c p sistor I32 togrid I33 of peak modulator tube I3. 5 tame Shunt between grids 11 and nd If the peaks of the signals impressed on grid I33 E tend to exceed the zero bias operating point for While Particular embodiments of the Present th tub m t, flows and biasing voltage invention have been shown and described, it will is produced ac r ist 135 which is m. be obvious to those skilled in the art that changes tained at an average value by the combination or and modifications m y e made without departcondenser 3 and resistor 35 As a m, of ing from this invention in its broader aspects,

the direct current re-insertion action just de- 161810176. the i i the pp n d claims scribed, the portion of the ignal lying below the is to cover all such changes and modifications as grid bias cut-oi! voltage increases until once fan within the true spirit and scope of this again the tips of the modulating signals lie at 5 ventmn' the zero bias level of operation for modulator I tube a l. The combination, in a device for transmit- In corresponding fa hio the signal of posmve ting signals having amplitude variations, of a polarity from plate 5 of am lifier tube 5 is source of carrier wave voltage, a pair of constant coupled through condenser I31 and grid resistor voltage amplifiers Connected t0 amplify separatem to grid no or carrier modulator time uio. v carrier wave voltage from d s ce, an im- The maximum Signal amplitude which corm pedance inversion network, a pair of carrier wave sponds to the gray region r th transmtted be? final amplifiers respectively driven by said convision picture, is maintained constant at the zero 5mm voltage amplifiers and having Output termi bias level of operation for modulator tube IlIl beinterconnected by said impedance inversion cause of t biasing action f resistor n and network, a common load connected to one end of condenser [31 in combination ,said network to which said final amplifiers sup- To assure 10w capacitance to ground on modu ply power additively, and means for grid modulated grids 1 5 19 and m final m lifi lating said carrier wave final amplifiers accordand 2, and hence. to minimize frequency dlstor- 40 m8 Said Variationst f th modulating signals a number of s 2. A high efiiciency transmitter of signals havcial features are provided in modulators Ill and mg amplitude Vaflations, incluiimg source of I carrier wave voltage; a pair of constant voltage First, the modulators are operated as cathode amplifiers each excited by Said source; followers which inherently assures low capaciphase Shifting network interposed between one fame to gmumi of said constant voltage amplifiers and said Second, the continuous voltage for screen grid source to cause Said to be excited) 3 m peak modulator tube '3 and for screen time phase quadrature; an impedance inversion and 3 in carrier modulator tube a B supplied network; a pair of final amplifiers. including from a low capacity power supply which vacuum tubes having plate, grid and cathode elecludes a high freq ency oscillator 5' operating merits, driven by said constant voltage amplifiers, at 15 kilocycles in a particular embodiment of for individually ampmymg over contiguous P this mvemiom peak power a lifi 5 and tions of the maximum amplitude range of said a ri power am lifier 1 receiving exciting signal variations, and interconnected by said imvoltage from oscillator H5, and peak and carrier Pedance inversion netwmk? a mad @(mnected o r tifi g d 9, respectively one end of said network, whereby said load re- Peak rectifier Mg incl d step-up transformer ceives power additiveiy from said final amplifier; I50, whi ha a, very l capacity s ground identical impedance step-down transformers recause so few turns are required on the transform- Spectively arranged to willie s constant volter to provide adequate inductive reactance at the e amplifi rs o said final amplifiers to efiect iniflh pply frequency. rectifier t across creased constancy of the driving voltage for said which the high voltage from transformer m is fi amplifiers; and means for modulating the impressed voltage regulator 53 and filter grid of each of said final amplifiers according to denser I53. The continuous voltage appearing said signals over respective ortions of the maxiacross filter condenser I53 is impressed between mum ampli u e range of said signals.

screen grid in and cathode IN of peak modula- A transmitter of signals h ving ampli ude m |3L Th tir Screen voltage upply thus variations including means for generating a carhas very l capacity t ground hi h d i rier wave and means for modulating the amplitude producing low capacity to ground for the d oi said carrier wave according to said variations, later as a whole. '70 said first means including a source of carrier Grid current meter I55 is connected between waves. a constant voltage amplifier connected to grid load resistor I56 and grid bias source I51, said source, a second constant voltage amplifier which results in the addition of minimum capaciand a 90 phase shifting network connected betanoe between grids I9 and of peak final amtween said second constant voltage amplifier and piifier tubes 83 and 85, and ground. I5 said source. a pair or impedance step-down trans.

formers. a pair 01' final amplifiers cathode counal amplifiers being connected to grid modulate pied to said constant voltage amplifiers through one or said carrier wave final amplifiers and the source of signal voltage, a direct current reinl0 tance voltage step-up transformer connected to separately driven by said direct current reinserthe capacitance shunted between said screen grid tion circuit, one 01' said signal amplifiers includelectrodes and ground is minimized. ing means for amplifying substantially only the 6. A device for supplying to a load signals havlower amplitude portion of said signal voltage, ing amplitude variations within a given amplitude the other of said amplifiers including means for range comprising: an impedance inversion netamplifying substantially only the remaining porwork connected to said load; a pair of amplifiers tion of said signal voltage, one of said signal amfor individually modulating and amplifying a plifiers being connected to grid modulate one 01' carrier wave signal in accordance with signal said final amplifiers and the other signal amplifier variations within an assigned portion of said being connected to grid modulate the remaining range and having output electrodes coupled to the final amplifier. opposite ends of said network for supplying sig- 4. In combination, in a transmitter of signals nals additively to said load through said network having amplitude variations, a carrier final amand separate constant voltage amplifiers conplifier and a peak final amplifier each adapted to nected to drive each of said employed carrier be modulated by said signal variations and each amplifiers from a arrier wave source.

driven by a constant voltage source, said car- 7. A transmitter of signals having amplitude rier and peak final amplifiers being connected variations including means for generating a carto supply power additively to a common load, and rier wav and means for modulating the amplibeing adjusted to amplify over contiguous lower tude of 5 id carrier wave according to said variand upper portions, respectively, or the maximum ations, said first means including a, source of caramplitude range of said signal variations, the rier waves, aconstant voltage amplifier connected impedance 01' said load tending to appear to said to said source, a second constant voltage amplifinal carrier amplifier as increasing as said peak fier and a 90 phase shifting network connected impedance inversion network including at least coupled o said constan vo t e amp a plifier becomes operative. 5 constant voltage amplifier, said means for modu 5. A transmitter of signals having amplitude lating said carrier wave generator including a variations including means for generating a carsource or signal voltage having variations withplifier connected to said source, a second conseparately driven by said direct current reinserstant voltage amplifier and a 90 phase shifting tion circuit, one of sai signal amplifiers includfinal amplifiers, and a load coupled to the plate 8. -A transmitter of signals having amplitude modulating said carrier wave generator including variations within a given maximum amplitude a pair of impedance step-down transformto substantially only a portion of the signal from ers; a pair of signal amplifiers, including vacusaid signal source and the other of said signal um tubes having grid, anode and cathode eleamplifiers including means for limiting the amments, cathode-coupled through said transformplification therein to substantially only the reers to said constant voltage amplifiers and opermaining portion 01' said signals, one of said sig- 7 ating with grids at a fixed reference potential for said carrier voltage; means for limiting amplification in each of said final amplifiers to an assigned one of different portions of said maximum amplitude range; an impedance inversion network interconnecting said anode elements of said final amplifiers; a load connected to said network for receiving power additlvely from said final amplifiers; and means for modulating the grid oi each oi said final amplifiers according to its assigned portion of said amplitude range of signal variations.

9. A transmitter of signals having amplitude variations within a given maximum amplitude range including: a source of carrier wave voltage; a pair of constant voltage amplifiers each excited by said source; a 90 phase shifting network in terposed between one of said constant voltage amplifiers and said source to cause said amplifiers to be excited in time phase quadrature; a pair of impedance step-down transformers; a pair of signal amplifiers, including vacuum tubes having grid, anode and cathode elements, cathode-coupled through said transformers to said constant voltage amplifiers and operating with grids at a fixed reference potential for said carrier voltage; an impedance inversion network interconnecting said anode elements of said final amplifiers; a load connected to said network for receiving power additively from said final amplitiers; and means for eii'ecting grid modulation of each of said final amplifiers inaccordance with an assigned one of diiierent portions of said amplitude range of signal variations.

10. A device for supplying modulated carrier wave signals to aload comprising: a source of carrier wave voltage; a pair of constant voltage amplifiers coupled to said source; a 90 phase shitting network interposed between one of said amplifiers and said source: a pair oi final amplifiers coupled to and driven by said pair of constant voltage amplifiers: an impedance inverter coupled between said final amplifiers and coupled to said load for supplying signals additively to said load from said final amplifiers; a source of modulating signals having amplitude variations within a given maximum amplitude range; means for translating diflerent portions of said range, including a pair 01. stabilizing networks for establishing a selected amplitude value of each oi said diflerent portions at a fixed reference potential; and a pair of modulators associated with said stabilizing networks and coupled to said final amplifiers ior eflecting modulation oi each final amplifier in accordance with the stabilized modulating signal of an assigned one of said portions of said range.

c JOHN F. BELL.

REFERENCES crrnn The following references are oi record in the file of this patent:

m s'ra'rns PATENTS 

